Thyroid and Parathyroid

the vagus to run directly to the larynx, often in close proximity to the superior thyroid
.( vessels, and may be at risk when these vessels are transected (Fig. 36-8
The superior laryngeal nerve arises from the vagus near the base of the skull and
descends medial to the carotid vessels. At the level of the hyoid bone it divides into
two branches, one sensory (internal branch), and the other motor (external branch).
The external branch runs on the lateral surface of the inferior constrictor muscle and
descends to innervate the cricothyroid muscle (Fig. 36-9). This muscle alters vocal
cord tension and affects the pitch of the voice. In most instances the nerve runs in
close proximity to the superior pole vessels, and in 21 percent of people it is closely
related to the vessels and is at significant risk if it is not identified at operation. To
avoid injury, the superior pole vessels should be individually ligated and divided low
. on the thyroid gland and dissected laterally to the cricothyroid muscle
PHYSIOLOGY
Through release of its principal hormones, thyroxine (T4) and triiodothyronine (T3),
the thyroid gland influences the metabolic rate of all tissues. Increased secretion
increases the metabolic rate; conversely, the rate decreases when secretion is
decreased. Release of T4 and T3 is stimulated by the anterior pituitary hormone
thyrotropin or thyroid- stimulating hormone (TSH). Secretion of TSH is directly
suppressed by T4 and T3 (a negative feedback loop). TSH release also is stimulated
by the hypothalamic hormone thyrotropin-releasing hormone (TRH). Thyroid
hormone production is influenced by numerous physiologic, pathologic, and
. pharmacologic factors
Iodine Metabolism
The formation of thyroid hormones is dependent on the availability of exogenous
iodine. The average daily iodine requirement is 0.1 mg. Iodine is found principally in
fish, milk, and eggs. In the United States, iodine is routinely added to bread and salt in
order to reduce the frequency of iodine deficiency. Iodine is rapidly converted to
iodide in the stomach and jejunum and is absorbed into the bloodstream within 1 h;
and from there it is distributed uniformly throughout the extracellular space (Fig. 36-
10). Iodide is actively transported into the thyroid follicular cells by an ATPdependent
process. The thyroid-serum iodine ratio under normal conditions is about
50:1, and most of the body's store of iodine is found in the thyroid gland (90 percent).
Thyroid-serum ratios can be as high as 500:1 in certain instances, such as iodine
. deficiency or Graves' disease
One-third of the loss of iodine from the plasma is accounted for by thyroid
concentration, and the other two-thirds through renal excretion. In studies involving
radiolabeled iodine, all the iodine is concentrated within the thyroid or excreted in the
urine within 48 h, and the plasma and tissues are mostly cleared of iodide. Evidence
of labeled iodine in serum is accounted for by secretion from the thyroid gland in the
. form of thyroid hormone
Synthesis of Thyroid Hormone
Steps in the synthesis of thyroid hormone are: (1) active trapping and concentration of
iodide in the follicular cell; (2) rapid oxidation of iodide to iodine; (3) linkage of
iodine with tyrosine residues in thyroglobulin; (4) coupling of these iodotyrosines
(monoiodo- and diiodotyrosine) to form the active thyroid hormones T4 and T3.